C0g multi-layered ceramic capacitor

ABSTRACT

A dielectric ceramic composition in a multilayer ceramic capacitor with a composition of formula: 
       {[(CaO) t (SrO) 1-t ] m [(ZrO 2 ) v (TiO 2 ) 1-v ]} 1-s-x A s E x            wherein:   A is a transition metal oxide;   E is an oxide of an element selected from the group consisting of Ge, Si, Ga and combinations thereof;   m is 0.98 to 1.02;   t is 0.50 to 0.90;   v is 0.8 to 1.0;   s and x are selected from the group consisting of: a) 0≦x≦0.08, 0.0001≦s≦0.043 and x≧1.86s; and b) 0≦x≦0.0533, 0.0001≦s≦0.08 and x≦0.667s.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of pending U.S. patent Ser.No. 11/273,548 filed Nov. 14, 2005 which, in turn, claims the benefit ofU.S. Provisional Patent Appl. No. 60/669,110 filed Apr. 7, 2007 both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

This application relates to ceramic capacitors having either a noblemetal or base metal electrode which conforms to the Electronics IndustryAlliance (EIA) Standard No. 198-1-F-2002 for temperature coefficientstandard C0G. U.S. Pat. No. 7,172,985 is also incorporated herein byreference.

BACKGROUND AND PRIOR ART

C0G capacitors have very low temperature drift Temperature Coefficientof Capacitance (TCC) (≦+/−30 ppm/° C.). Typically, the primarycomponents of the ceramic include magnesium titanate or barium neodymiumtitanate.

The use of base metal electrodes such as Ni, Cu, and 80 Ni:20 Cu forcapacitors offer significant material cost advantages over noble metalsor precious metal electrodes such as Pt, Pd, Au, Ag and combinationsthereof. Ni and Cu are conductive, comparatively inexpensive metalswhich, in pure form, are not facilely oxidized. Both can be deposited aselectrodes using screen printing processes on the same equipmentconventionally used for depositing noble metals. Ni has a higher meltingpoint (Ni mp 1450° C.; Cu mp 1083° C.—Weast Handbook of Chemistry &Physics, 46^(th) edition) and is preferred for multi-layered ceramiccapacitors (MLCC) fired at higher temperatures.

While the ceramic dielectrics of this invention may be used withprecious metals to obtain C0G MLCC capacitors (which may be fired inoxidative environments), BME capacitors are preferred.

Numerous compositions have been disclosed for non-reducing typedielectric ceramic compositions including U.S. Pat. Nos. 5,204,301;6,118,648; 6,295,196; 6,329,311; 6,387,835; 6,396,681; 6,327,311;6,525,628; 6,572,793; 6,645,897; 6,656,863; 6,858,554 and 7,172,985 aswell as published patent application numbers US 2005/0111163; US2003/0186802 and US 2004/0220043. These disclosures are directed tovarious combinations of Ca, Sr, Zr, Ti and Ba oxides with or withoutlimited amounts of dopant oxides or alkaline, alkaline earth and rareearth metals wherein individual precursors are fired to form a ceramicmatrix. These ceramics, though beneficial, are still inferior withregards to overall capacitor performance. There has been an ongoingeffort in the art to provide a capacitor with improved properties and,specifically, to ceramics which can provide an improved capacitor.

BRIEF DESCRIPTION OF THE INVENTION

It is a first objective of this invention to provide a Base MetalElectrode (BME)

Multilayer Ceramic Capacitor (MLCC) device having a high CV (capacitanceper unit volume).

It is a second objective of this invention to produce an MLCC devicewhich meets the C0G specification for Temperature Coefficient ofCapacitance (≦+/−30 ppm/° C.).

It is a further objective of this invention to provide a MLCC capacitormeeting C0G specifications which can be produced at a price competitivewith lower performing devices such as those meeting C0H, C0J, C0K, SL,R2J, X7R, etc., and lower specifications, and which meet industrystandards for reliability.

These and other advantages are provided in a dielectric ceramiccomposition in a multilayer ceramic capacitor with a composition offormula:

{[(CaO)_(t)(SrO)_(1-t)]_(m)[(ZrO₂)_(v)(TiO₂)_(1-v)]}_(1-s-x)A_(s)E_(x)

wherein:

-   -   A is a transition metal oxide;    -   E is an oxide of an element selected from the group consisting        of Ge, Si, Ga and combinations thereof;    -   m is 0.98 to 1.02;    -   t is 0.50 to 0.90;    -   v is 0.8 to 1.0;    -   s and x are selected from the group consisting of: a) 0≦x≦0.08,        0.0001≦s≦0.043 and x≧1.86s; and b) 0≦x≦0.0533, 0.0001≦s≦0.08 and        x≦0.667s.

Yet another embodiment is provided in a method for forming a capacitorcomprising:

a) milling to a D50 of less than 1.0 μm a material comprising:

[(CaO)_(t)(SrO)_(1-t)]_(m)[(ZrO₂)_(v)(TiO₂)_(1-v)]

-   -   wherein m is 0.98 to 1.02; and    -   t is 0.50 to 0.90; and    -   v is 0.8 to 1.0;    -   thereby forming a first component (C1);        b) milling MnO₂, MnCO₃ or another oxidized form of Mn to a D50        of less than 1.0 μm thereby forming a second component (C2);        c) milling SiO₂ to a D50 of less than 0.50 μm thereby forming a        third component (C3);        d) combining the first component, the second component and the        third component with a solvent in a ratio C1_(1-α-β)        C2_(α)C3_(β) thereby forming a coating solution;        e) applying said coating solution to a tape in an amount        selected from the group consisting of a) sufficient to form a        ceramic layer over 4.5 μm thick wherein α and β are defined as        0≦β≦0.08, 0.0001≦α≦0.043 and β≧1.86α; and b) sufficient to form        a ceramic layer of no more than 4.5 μm thick wherein α and β are        defined as 0≦β≦0.0533, 0.0001≦α≦0.08 and β≦0.667α;        f) drying the coating solution to form a green coating;        g) depositing an ink comprising electrode material over said        green coating to form electroded green layers;        h) stacking non-electroded green layers and electroded green        layers to form a green capacitor stack;        i) dicing the capacitor stack to form singular green multilayer        chips;        j) firing the singular green multilayer chips in an atmosphere        with a PO₂ of 10⁻⁶ to 10⁻¹⁶; and        k) forming terminals in electrical contact with the electrode        material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a multilayer ceramic capacitor according tothis invention.

DETAILED DESCRIPTION OF THE INVENTION

The use of base metals as the conductive metal in a capacitor electrodeallows the performance in the capacitor to be maintained whiledecreasing materials cost. FIG. 1 is a side view of a conventionalmulti-layer or stacked ceramic capacitor 1. Conductive plates 3, 5 serveas electrodes and are connected to terminations 7, 9 in alternatingorder. The electrodes are separated or isolated by dielectric ceramic11. A resin, 12, encases a portion of the capacitor as known in the art.

The electrodes 3, 5 may be made from any conductive material but aretypically noble metals such as Pt, Pd, Au or Ag. Since noble metals aredifficultly oxidized, when the green stacked plates are fired, hightemperatures and an oxidizing atmosphere may be used, and a ceramichaving a high dielectric constant is obtained. Good temperaturecoefficients of capacitance may be obtained.

The use of base metals requires modifications in the composition of theceramic and in the conditions of firing. Formulations are desired whichhave a low Temperature Coefficient of Capacitance (TCC), preferablymeeting the EIA C0G standard (≦+/−30 ppm/° C.).

Through diligent research a complex relationship between composition anddielectric thickness has been determined. In general, at a ceramicthickness above 4.5 μm a composition which is relatively rich in anoxide of group III or group IV demonstrates a significant increase incapacitor performance and at a ceramic thickness below 4.5 μm acomposition which is relatively rich in transition metal oxidedemonstrates a significant increase in capacitor performance as morethoroughly discussed herein.

Preferred ceramics are defined according to formula (I).

{[(CaO)_(t)(SrO)_(1-t)]_(m)[(ZrO₂)_(v)(TiO₂)_(1-v)]}_(1-s-x)A_(s)E_(x)  (1)

In formula (1) A is a transition metal oxide preferably selected fromCu, Mn, Mo, W, Co, Ta, Sc, Y, Hf, V, Nb, Cr and combinations thereof;Most preferably A is manganese oxide. E is an oxide of a group III or IVelement selected from Ge, Si, Ga and combinations thereof. Subscripts informula (1) have the following values: m is 0.98 to 1.02; t is 0.50 to0.90 and v is 0.8 to 1.0. When the ceramic is applied at a thicknessabove 4.5 μm the subscripts s and x are defined as 0.0001≦s≦0.043;0≦x≦0.08 and x≧1.86s. When the ceramic is applied at a thickness of 4.5μm or less, the subscripts s and x are defined as 0.0001≦s≦0.08;0≦x≦0.0533 and x≦0.667s.

The compound of formula (1) is unique in that a precursor materialdefined as [(CaO)_(t)(SrO)_(1-t)]_(m)[(ZrO₂)_(v)(TiO₂)_(1-v)] is mixedwith an appropriate amount of a precursor of a dopant oxide. The methodtypically employed in the art includes the firing of a mixture of oxideprecursors, such as carbonates, thereby forming a single phase of aprimary material and secondary phases dependant on ratios of reactantsand the phase compositions. Oxide precursors are materials which are anoxide after heating as described herein. Particularly preferred oxideprecursors include oxides, carbonates, oxalates, peroxides, acetates,nitrates and the like. In the present application the primary phase ispredetermined as the (Ca,Sr)(Zr,Ti)O₃ material and dopants are addedthereto which, presumably, form phases differing from that formed byfiring precursors of the oxides of calcium, strontium, zirconium,titanium and dopants. As well known to those of skill in the art, minorvariations in composition, either globally or locally, can result inphases which are neither predictable nor controllable. Therefore, withthe prior art techniques there may be unintentional secondary phasesformed and these vary from batch to batch and therefore from capacitorto capacitor. The material prepared herein provides a ceramic withgreatly improved consistency and provides unpredicted advantages withregards to C0G relative to ceramic materials formed in accordance withthe prior art.

A particularly preferred formulation is provided with a base material of(CaO)_(0.7)(SrO)_(0.3)(ZrO₂)_(0.97)(TiO₂)_(0.03) which is preferablydoped with one or more of MnO, MnO₂, MnCO₃, SiO₂. All formulations aremilled at the slurry or slip stage in a suitable milling solution suchas water, alcohol, toluene or a combination thereof, or dihydroterpinol(DHT) or other suitable milling solutions using suitable media to a sizeof D₅₀ ca. <1.0 μm or less. The slip is spread on a carrier filmmaterial using a doctor blade. The electrodes are preferably depositedvia screen printing using a conductive ink. The chips are laminated,diced, burned out and fired in a reducing atmosphere of PO₂ equal toabout 10⁻⁶ or less. Soak temperatures from 1200° C. to 1350° C. may beselected.

C0G ceramic capacitors can be made using the mole % of MnO₂ and SiO₂present in amounts between 0 and ˜8 mole %. When the ceramic is appliedat a thickness above 4.5 μm the mole ratio of MnO₂ is at least 0.01 molepercent to no more than 4.3 mole percent and the SiO₂ is present in nomore than 8 mole percent with the proviso that the mole percent of SiO₂is at least 1.86 times the mole percent of MnO₂. When the ceramic isapplied at a thickness of 4.5 μm or less, the mole ratio of MnO₂ is atleast 0.01 mole percent to no more than 8 mole percent and the SiO₂ ispresent in no more than 5.33 mole percent with the proviso that the molepercent of SiO₂ is less than 0.667 times the mole percent of MnO₂.

The preparation of laminated ceramic capacitors is well documented andthe present invention does not alter the manufacturing process to anysignificant degree relative to standard procedures known in the art.

As an example of a manufacturing process, ceramic slurry is prepared byblending and milling the ceramic compounds described herein with adispersant in either water or an organic solvent such as, for example,ethanol, isopropanol, toluene, ethyl acetate, propyl acetate, butylacetate, mineral spirits or other suitable hydrocarbon liquid, or ablend thereof. After milling, a ceramic slip is prepared fortape-casting by adding a binder and a plasticizer to control rheologyand to give strength to the tape. The obtained slip is then processedinto a thin green sheet by tape-casting on a flat polymer film. Afterdrying the sheet, a multiplicity of electrodes are patterned on thesheet by using, for example, a screen-printing method to form a printedceramic sheet.

A laminated green body, or stack, is prepared by stacking onto asubstance such as polycarbonate, polyester or a similar method: 1) acertain number of unprinted ceramic sheets representing the bottomcovers, then 2) a certain number of printed ceramic sheets in alternatedirections so as to create alternating electrodes that terminate atopposing ends, and 3) a certain number of unprinted ceramic sheetsrepresenting the top covers. Variations in the stacking order of theprinted and unprinted sheets can be used with the dielectric material ofthis invention. The stack is then pressed at between 20° C. and 120° C.to promote adhesion of all laminated layers.

The laminated green body is then cut into individual green chips.

The green chip is heated to remove the binder. The binder can be removedby heating at about 200-700° C. in atmospheric air or neutral orslightly reducing atmosphere for about 0.5 to 48 hours.

The dielectric is then sintered in a reductive atmosphere with an oxygenpartial pressure of 10⁻⁶ to 10⁻¹⁶ atm at a temperature not to exceed1350° C. The preferred temperature is about 1,200 to 1,325° C. Aftersintering the dielectric is reoxidized by heating to a temperature of nomore than about 1,100° C. at an oxygen partial pressure of about 10⁻⁵ to10⁻¹⁰ atm. More preferably, the reoxidation is done at a temperature ofabout 800 to 1,100° C. The material resulting from this stage istypically referred to as a sintered chip.

The sintered chip is subjected to end surface grinding by barrel or sandblast, as known in the art, followed by transferring outer electrodepaste to form the external electrodes. Further baking is then done tocomplete the formation of the outer electrodes. The further baking istypically done in nitrogen or slightly oxidizing atmosphere at atemperature of about 600-1000° C. for about 0.1 to 1 hour.

Layers of nickel and tin or other suitable solder composition can thenbe plated on the outer electrodes to enhance solderability and preventoxidation of the outer electrodes.

Example 1

A base formulation of (CaO)_(0.7)(SrO)_(0.3)(ZrO₂)_(0.97)(TiO₂)_(0.03)was mixed into the milling solution and milled in an horizontal beadmill with 1 mm spherical media to D₅₀=0.35 μm. Separately MnO₂ and SiO₂were mixed with milling solution and milled to less than ca. 0.4 μmusing 1 mm media in a jar mill. The MnO₂ and SiO₂ were mixed with thebase formulation. Tapes were coated via a tape caster using a doctorblade for a target fired dielectric thickness of about 6 μm. The Nielectrodes were deposited via screen printing using suitable ink. Afterlamination of a green stack and dicing to achieve singular greenmultilayer chip devices, the singular MLCCs were submitted to a thermalburnout process to remove the organic material. The chips were fired at1305° C. and 1325° C. respectively, in a reducing atmosphere of about10⁻⁶ to 10⁻¹⁶ atm O₂. The chips were subsequently corner rounded andterminated with a suitable copper termination. Similar chips were madewith MnO₂ from 0 to 6.9 mol % and SiO₂ from 0 to 6.9 mol %, as shown inTable 1. Comparisons of the physical properties as a function ofcomposition and firing temperature are shown in Table 1. Capacitance anddissipation factor (DF) were measured at 1V, 1 MHz with a HP4275A LCRmeter on 30 samples for each composition. Temperature coefficient ofcapacitance (TCC) was measured at −55° C. and 125° C. at 1V, 1 MHz witha Saunders & Associate chamber (6 samples each). Only the largest valuesof TCC between −55° C. and 125° C. are reported in Table 1. Insulationresistance was measured at 50V DC at 25° C. and 125° C. in a Saunders &Associate chamber (6 samples). The 25° C. were all very high, in the TΩrange, and are not reported in Table 1. Breakdown voltage (UVBD) wasmeasured using a H100DC tester (Rod-L Electronics, Inc.).

TABLE 1 Compositions and Electrical Properties of Example1 Samples.Firing UVBD Batch [MnO2] [SiO2] Temperature Capacitance DF TCC Max IRTest 125° C. 125° C. Average # mol % mol % (° C.) (nF) (%) (ppm/° C.)Voltage IR (GΩ) RC (ΩF) (V) 15 0.00% 0.00% 1305 2739 0.026 −28 50 45 122628 6 0.91% 8.15% 1305 2465 0.087 15 50 456 1087 513 5 0.95% 4.25% 13052522 0.094 −4 50 385 956 494 1 0.97% 2.17% 1305 2646 0.027 3 50 130 345618 7 0.98% 0.98% 1305 2673 0.107 −19 50 260 686 583 2 1.93% 2.15% 13052746 0.052 −6 50 282 726 772 8 3.72% 3.72% 1305 2641 0.027 9 50 218 561821 3 3.78% 2.11% 1305 3131 0.017 7 50 192 580 633 9 6.92% 6.92% 13052303 0.042 34 50 200 442 442 15 0.00% 0.00% 1325 2818 0.018 −27 50 3641003 599 6 0.91% 8.15% 1325 2470 0.144 17 50 429 1027 440 5 0.95% 4.25%1325 2576 0.080 −3 50 450 1140 531 1 0.97% 2.17% 1325 2585 0.037 −7 50376 973 580 7 0.98% 0.98% 1325 2649 0.091 −24 50 370 969 501 2 1.93%2.15% 1325 2660 0.102 −7 50 412 1029 570 8 3.72% 3.72% 1325 2586 0.030 850 399 1004 798 3 3.78% 2.11% 1325 2683 0.019 6 50 296 766 9 6.92% 6.92%1325 2434 0.053 −1 50 313 732 493

As seen in Table 1, high values of capacitance and low DF were achievedwith most samples. All but one sample gave TCC within the C0Gspecification. IR are well above the C0G specification (125° C. RCproduct >100 Ω.F).

Next, reliability performance of the present samples was measured and issummarized in Table 2. In Table 2, Si and Mn represent the mole fractionof each component. TH is the fired dielectric thickness in μm. Caprepresents relative capacitance. DF is a relative dissipation factor. IRis relative insulation resistance. HALT is the results of a HighlyAccelerated Life Test done for 92 hours at 175° C. under 8 times ratedvoltage wherein a rating of “good” indicates a mean time to failure(MTTF) of longer than 3000 minutes, “medium” represents an MTTF oflonger than 500 minutes but shorter than 3000 minutes and “poor”represents an MTTF of less then 500 minutes. Life Test was conducted for1000 hours at 2 times rated voltage at 125° C. on 77 pieces per batch.The measurement criterion is insulation resistance of the pieces beforeand after the test wherein a rating of “good” indicates zero pieces faildue to degradation of insulation resistance to below the IR limitrecommended by Electronics Industry Association (EIA), “medium”indicates 1 to 3 pieces out of 77 pieces failed and “poor” indicatesmore than 3 pieces of 77 failed. Humidity Test was conducted for 1000hours at the rated voltage at 85° C. at 85% relative humidity and thecriteria are the same as for the Life Test.

As shown in Table 2, good reliability (all three of HALT, Life Test andHumidity Test performance) is only obtained for Si between 0 and 0.08,Mn between 0.0001 and 0.043 and Si≧1.86*Mn.

TABLE 2 Humidity Si Mn TH Cap DF IR HALT Life Test Test 01* 0.01 0.01 6Good Medium Medium Poor Poor Very Poor 02* 0.01 0.02 6 Good Good GoodPoor Poor Poor 03* 0.02 0.02 6 Good Good Good Medium Medium Poor Present0 ≦ Si ≦ 0.08 0.0001 ≦ Mn ≦ 0.043 6 Good Good Good Good Good GoodInvention AND Si ≧ 1.86*Mn *Compositions outside the scope of thepresent invention.

Capacitors of the type disclosed herein may be substituted for polymerfilm capacitors, Al, Nb and Ta capacitors, or for existing noble metalor base metal electrode based MLCC capacitors. Both lower costs andsuperior TCC are possible in this family of formulations.

Example 2

A series of ceramic formulations comprising(CaO)_(0.7)(SrO)_(0.3)(ZrO₂)_(0.97)(TiO₂)_(0.03) and additions of SiO₂and MnO₂ was prepared as set forth in Table 3. The compositions werecoated to target a fired dielectric thickness of 3 μm and the physicalproperties were measured as in Example 1 and recorded in Table 3.

TABLE 3 Humidity Si Mn TH Cap DF IR HALT Life Test Test 04* 0.01 0.01 3Good Good Very Poor Poor Poor Very Poor 05* 0.01 0.02 3 Good Good PoorPoor — Poor 06* 0.02 0.02 3 Good Good Poor Poor — Poor 07 0.01 0.04 3Good Good Good Good Good Good 08 0.02 0.04 3 Good Good Good Good GoodGood Present 0 ≦ Si ≦ 0.053 0.0001 ≦ Mn ≦ 0.08 3 Good Good Good GoodGood Good Invention AND Si ≦ 0.6667*Mn *Compositions outside the scopeof the present invention.

As indicated in Table 2 and 3 the results are dependent on thecomposition and dielectric thickness of the MLCC. At intermediate ratiosof transition metal oxide and Group III/IV oxide the performance of theresultant capacitor is diminished relative to the present inventionrange.

The invention has been disclosed in consideration of specific exampleswhich do not limit the scope of the invention. Modifications apparent toone having skill in the art subsumed within the scope of the invention.

1-16. (canceled)
 17. A capacitor formed by the method of milling to aD50 of less than 1.0 μm a material comprising:[(CaO)_(t)(SrO)_(1-t)]_(m)[(ZrO₂)_(v)(TiO₂)_(1-v)] wherein m is 0.98 to1.02; and t is 0.50 to 0.90; and v is 0.8 to 1.0; thereby forming afirst component (C1); milling MnO₂, MnCO₃ or another oxidized form of Mnto a D50 of less than 1.0 μm thereby forming a second component (C2);milling SiO₂ to a D50 of less than 0.50 μm thereby forming a thirdcomponent (C3); combining said first component, said second componentand said third component with a solvent in a ratioC1_(1-α-β)C2_(α)C3_(β) thereby forming a coating solution; applying saidcoating solution to a tape in an amount selected from the groupconsisting of a) sufficient to form a ceramic layer over 4.5 μm thickwherein α and β are defined as 0<β<0.08, 0.0001<α<0.043 and β>1.86α; andb) sufficient to form a ceramic layer of no more than 4.5 μm thickwherein α and β are defined as 0<β<0.0533, 0.0001<α<0.08 and β<0.667α;drying said coating solution to form a green coating; depositing an inkcomprising electrode material over said green coating to form electrodedgreen layers; stacking non-electroded green layers and electroded greenlayers to form a green capacitor stack; dicing said capacitor stack toform singular green multilayer chips; firing said singular greenmultilayer chips in an atmosphere with a PO₂ of 10⁻⁶ to 10⁻¹⁶; andforming terminals in electrical contact with said electrode material.18. The capacitor according to claim 17 which has a temperaturecoefficient of capacitance of ≦+/−30 ppm/° C.